Recent reports of electrically-pumped lasing and optical gain in organic semiconductor crystals and optically pumped lasing in amorphous semiconducting thin films have encouraged the pursuit of practical organic semiconductor lasers. Organic thin-films accommodate low-cost fabrication, and can exploit the wide variety of dye molecules in inexpensive organic lasers that are tunable across the visible spectrum. These properties may ultimately enable organic materials to find practical application in electrically-pumped lasers.
Conventional semiconductor lasers use covalently-bonded crystalline materials. One reason for the success of these devices is that the photoluminescence efficiency of a direct-energy-gap crystalline semiconductor such as GaAs is approximately 100%, independent of excitation strength. Organic light emitting devices (OLEDs) have also demonstrated a maximum internal efficiency approaching 100%. However, the highest efficiency organic electroluminescence is typically realized at low current densities because non-radiative losses increase under high injection. At the excitation densities required for electrically-pumped lasing, such losses might render organic lasers impractical.
Previously, the inability to realize electrically-pumped lasers using amorphous organic films was attributed to the optical absorption of injected charge, or ‘polaron absorption.’ In the following description, singlet-singlet, singlet-polaron, and singlet-triplet quenching, and electric field-induced exciton dissociation, are examined. Singlet-singlet annihilation may prevent the achievement of lasing in crystalline materials, unless it can be overcome. In amorphous films, triplets induce losses that can be minimized by high rates of triplet-triplet and triplet-polaron annihilation, and in films that employ Förster energy transfer between host and guest molecules, singlet-polaron quenching limits the quantum efficiency.
The effect of all non-radiative processes is quantified by the external quantum efficiency-current density product, ηEXTJ. Since singlet-polaron quenching is the non-radiative analog of polaron absorption, an intensity independent quantum efficiency is associated with low polaron absorption. Bimolecular non-radiative losses such as singlet-polaron annihilation increase with excitation strength, thus amorphous thin film laser structures must be designed to operate at threshold current densities JTH<1000 A/cm2.
As a prototype crystalline laser, the tetracene device of Schön et al. is discussed in detail herein. Although electrically-pumped lasing has not been convincingly demonstrated, significant spectral narrowing suggests that lasing may be achievable with an improved device structure. The bulk tetracene crystals used in this device possess low optical confinement making it difficult to account for the observation of spectral narrowing at the reported threshold current density. Consequently, the inventors examined the possible effects of electron hole plasmas (EHPs) and defect or interface initiated self-focusing, and found that the formation of an EHP replaces the dominance of excitonic effects in the optical properties of organic semiconductors with more band-like characteristics. This, in turn, reduces or even eliminates multi-exciton annihilation that presents a significant barrier to gain at lower excitation densities.
The observation of electron-hole plasmas (EHPs) is a common occurrence in both direct and indirect inorganic semiconductor materials. In fact, in III-V semiconductor lasers, light emission can be a direct result of the presence of an EHP that is formed within the device. EHPs can result from extremely high densities of charges and excitons present near an interface, or confined in a heterostructure. An EHP can be formed when the density of electron-hole pairs, ncp, is greater than approximately 1/aB3, where aB is the Bohr radius of the excitons. Accordingly, 1/aB3 can be referred to as the “critical density” for the formation of an EHP. For organic materials, an exciton is often no larger than a molecular diameter (i.e., on the order of 10 Å), implying critical densities on the order of the molecular density.
In crystalline devices, the interfacial charge density in the exciton formation zone can reach 1013 cm−2. That is, the density of charges may approach the density of molecular sites. Under such conditions, free carriers screen Coulomb interactions within the excitons themselves. In addition, phase space filling of the bands at the interface excludes some electronic states required to complete excitonic wavefunctions. Both screening by free carriers and phase space filling prevent exciton formation, and lead to the creation of an EHP. For an organic molecular crystal with localized excitons, the exciton density required for the formation of an EHP (which, as described above, is related to the size of the excitons) is expected to approach the density of molecular sites.
One consequence of the formation of an EHP is the disappearance of the quasi-particle nature of discrete excitons. This in turn eliminates many excitonic processes such as multi-exciton annihilation and spin-selective radiative recombination in favor of direct band-to-band recombination at an interface or other region where excitons and carriers could localize at high densities, such as, for example, a region surrounding a defect or inside a heterostructure.
The prior art has suggested a light-emitting field effect transistor in which excitons are generated, leading to radiative recombination (see Schön, et al, A Light-Emitting Field Effect Transistor). Above a certain threshold, Schön teaches, coherent light is emitted through amplified spontaneous emission.
FIG. 1 is a schematic picture of an ambipolar FET 100 according to Schön under balanced electron and hole injection (i.e., Vg≈½ Vd,). Whereas electrons are accumulated near the source electrode 112, the negative bias of the gate 120 with respect to the drain 114 results in the accumulation of positive charge carriers close to the drain region. Assuming similar mobilities for both types of charge carriers in the organic layer 116, as in the case of α-6T, more or less balanced electron and hole injection is found at a gate bias of Vg≈½ Vd, which is high compared to the threshold voltages for n- or p-channel activity. In the case of a positive gate bias, electrons are accumulated near the source electrode 112. However, close to the drain region, the gate bias with respect to the drain 114 is negative, resulting in the accumulation of positive charge carriers. Consequently, a pn-junction is formed in the channel region 118 of the transistor 100. The two carrier types flow toward each other, excitons are generated, and radiative emission from the transistor 100 is observed. Hence, according to Schön, the FET 100 works as an LET.
The device of Schön, however, does not teach or suggest the formation of an EHP within the device. It would be advantageous, therefore, if there were available organic light emitting devices having compositions and geometries that are suitable to provide lasing based on the formation of an electron hole plasma within the device.